Chemically defined and semi‐defined media were designed for the preservative‐efficacy testing micro‐organisms designated by the United States Pharmacopoeia, in which the organisms went into the stationary phase of growth at an optical density (E470) of 1.0, because of depletion of a single carbon, nitrogen or phosphate source.Aspergillus nigerwas grown on solid media containing concentrations of these nutrients which limited the rates of mycelial development and sporulation density. The ability of the micro‐organisms to survive and grow in the presence of chlorhexidine diacetate, benzalkonium chloride and thiomersal varied markedly with the nutrient‐depletion of the inocula. No universal pattern of sensitivity emerged among microorganisms. OnlyA. nigershowed little overall change in preservative sensitivity. These results highlight the need to define more adequately growth media and conditions for the production of inocula for antimicrobial challe
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